X-RAY TOMOGRAPHY OF SULFIDE/SILICATE INTERFACE SUBJECTED TO AN ELECTRIC FIELD AT 20 KBAR/1400C

Investigating dynamic Earth processes such as physical segregation and chemical evolution of melts requires increasingly intricate petrology experiments, often performed under non-equilibrium conditions. Correspondingly, more sophisticated characterization methods are essential to quantify and interpret the results in terms of Earth processes. Here we present a joint investigation of dynamic electrochemical processes at a silicate/sulfide melt interface using state of the art experimental petrology experiments and X-ray computed micro-tomography (CT).

A source or sink of electrons at a silicate/metal interface can create a wide range of physical phenomena and chemical reactions, including changing surface energy and wetting behavior, altering the mineral phase stability, driving the partitioning behavior of major and minor elements between the metal and silicate, and locally perturbing the effective oxygen fugacity (e.g. Kavner and Walker 2004; Kavner et al., 2006). To investigate this rich array of dynamic phenomena at high pressure and temperature conditions relevant to the deep crust and upper mantle, three electrochemical experiments on a molten silicate/sulfide metal interface were performed in a piston cylinder apparatus at 10-30 kbar and 1400-1600 K. For each experiment, a 1V potential difference was applied between electrodes embedded in a basalt composition silicate and adjacent sulfide melt with positive, neutral, and reverse polarity.

A recently acquired high resolution X-ray tomography machine at the American Museum of Natural History was used to take three-dimensional images of the recovered samples. The CT-scans reveal structural details that were invisible in the previously obtained 2-di electron micrographs. Whereas the microprobe analysis revealed a zone of Rh-rich oxide phase adjacent to the Pt-10%Rh anode in the silicate, the CT scan reveals a three-dimensional plume-like structure emanating from the anode and dissipating as it approaches the sulfide cathode. Three-dimensional structures are also observed in the reverse-polarity sample, and are absent in the control sample. We interpret the structures in terms of dynamic behavior at a redox-active sulfide/silicate molten interface.